A reciprocating-subunit, or flip-flop, mechanism has been proposed to explain evidence for subunit interaction in two enzymes, horse-liver alcohol dehydrogenase and pig-heart mitochondrial malate dehydrogenase, and is relevant to the function of any homo-oligomeric enzyme. The research is intended to test subunit reciprocation in these enzymes plus two others for which there is published evidence of subunit interaction, through purification and kinetic study of enzyme molecules that have been alkylated at only half of their active sites. A novel, doubly charged, alkylating agent, 3-bromo-2-oxo-propyl phosphonic acid, would be used, because of its demonstrated extreme reactivity with alcohol dehydrogenase and because of the possibility of applying ion-exchange chromatography or isoelectric focusing to purify partially modified enzyme molecules.